1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display panel having a light impermeable layer with alignment marks. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing a misalignment of color filters.
2. Discussion of the Related Art
A flat panel display device has been widely used because it is thin and light in weight and requires low power consumption. The flat panel display device may be classified into two types by light emission. One is a light-emitting display device that emits light to display images and the other is a light-receiving display device that uses an external light source to display images. Plasma display panels (PDPs), field emission display (FED) devices, and electro luminescence (EL) display devices are examples of the light-emitting display devices, and a liquid crystal display device is an example of the light-receiving display device. The liquid crystal display device has been widely used for laptop computers and desktop monitors because of its superiority in resolution, color image display, and quality of displayed images.
Generally, the liquid crystal display (LCD) device has upper and lower substrates, which are spaced apart and facing into each other. Electrodes formed on the substrates are facing into each other. A liquid crystal is interposed between the upper substrate and the lower substrate. A voltage is applied to the liquid crystal through the electrodes of each substrate, and thus an alignment of the liquid crystal molecules is changed in accordance with the applied voltage to display images. Because the liquid crystal display device does not emit light as described above, it needs a light source to display images. Accordingly, the liquid crystal display device has a backlight behind a liquid crystal panel as a light source. An amount of incident light from the backlight is controlled in accordance with the alignment of the liquid crystal molecules to display images.
FIG. 1 is a schematic cross-sectional view illustrating a pixel of a conventional LCD panel having a thin film transistor as a switching device. As shown, the LCD panel has lower and upper substrates 10 and 50 and a liquid crystal (LC) layer 30 interposed therebetween. The lower substrate 10 has a thin film transistor (TFT) “T” as a switching element that switches a voltage for changing an orientation of the LC molecules. The thin film transistor “T” includes a gate electrode 12 on a transparent substrate 5, an active layer 16, first and second ohmic contact layers 18a and 18b, and source and drain electrodes 20a and 20b. A gate insulation layer 14 formed on the transparent substrate 5 is disposed between the gate electrode 12 and the active layer 16. The lower substrate 10 includes a passivation layer 22 to cover the TFT “T”. The lower substrate 10 also includes a pixel electrode 26 on the passivation layer 22, which is used to apply electric fields across the LC layer 30 in response to signals applied to the TFT “T”. The gate electrode 12 and the source electrode 20a are connected to a gate line and a data line, respectively, which perpendicularly cross each other and define a pixel region, i.e., a display area. The pixel electrode 26 is disposed in the pixel region and contacts the drain electrode 20b through a drain contact hole 24.
The upper substrate 50 includes a transparent substrate 5 and a plurality of color filters 54 on the surface of the transparent substrate 5 facing into the lower substrate 10 for representing colors. A black matrix 52 is disposed between the transparent substrate 5 and the color filters 54 and corresponds to the TFT “T”. An overcoat layer 56 is formed on the color filters 54 such that the overcoat layer 56 protects the color filters 54 and makes the surface be planar. A common electrode 58 formed of a transparent conductive material is formed on the overcoat layer 56. The common electrode 58 serves as an electrode that produces electric fields across the LC layer with the pixel electrode 26.
Although FIG. 1 only shows one TFT “T”, the lower substrate 10 usually includes a plurality of TFTs as well as a plurality of pixel electrodes contacting one another. The lower substrate 10 and the upper substrate 50 are respectively formed through different processes and then attached to each other. As mentioned before, the liquid crystal display device is a light-receiving display device, so that a backlight as a light source is necessary behind the liquid crystal panel. The light generated from the backlight passes through the upper and lower substrates and the liquid crystal layer. A light path is changed by an alignment of the liquid crystal molecules. Namely, the liquid crystal display device has a backlight as a light source behind the liquid crystal panel and an amount of the incident light from the backlight is controlled in accordance with the alignment of the liquid crystal molecules to display images.
Further, the black matrix 52 corresponds to the TFT “T”, in FIG. 1. The black matrix 52 is disposed at the peripheries of the transparent substrate 5 of the upper substrate 50 and forms a rectangular shape having a display region therein. The display region is divided into a plurality of parallel stripes so that the color filters 54a, 54b, and 54c (shown in FIGS. 4 and 5) are positioned at each parallel stripe. The black matrix 52 on the upper substrate 50 usually has a rectangular shape to surround all the color filters and divide each color filter into a stripe shape.
In forming the upper substrate 50 (shown in FIG. 1), the black matrix 52 is formed on the transparent substrate 5, and then the color filters 54 representing red (R) 54a, green (G) 54b, and blue (B) 54c colors (shown in FIG. 4) are formed on the transparent substrate 5 to cover the black matrix 52. The color filters 54 are arranged in a repeated order of red (R) 54a, green (G) 54b, and blue (B) 54c, for example, and each red, green, and blue color filter corresponds to the pixel electrode 26 of the lower substrate 10. Thus, full color images is accomplished by the combination of the R, G, and B color filters 54a, 54b, and 54c. In other words, each R, G, and B color filter 54a, 54b, and 54c corresponds to each pixel electrode 26 located in the pixel region, and the light passing through these R, G, and B color filters displays various colors.
As mentioned before, the liquid crystal display device is a light-receiving display device, so that it requires a backlight device to display images. Namely, the liquid crystal display (LCD) panel transmits image data using the light emitted from the backlight device positioned behind the LCD panel. However, the light emitted from the backlight device is almost absorbed by the LCD panel while passing through the lower substrate 10, the liquid crystal layer 30 and the upper substrate 50. Therefore, only 3 to 8% of the incident light generated from the backlight device can be employed in the LCD device. As a result, the LCD device provides inefficient optical modulation.
With the result of the poor light efficiency, various attempts have been made to improve the light efficiency of the liquid crystal display device. One of the efforts is to change a stripe shaped black matrix that divides the striped color filters into an island shaped black matrix, so that an aperture ratio of the LCD panel increases. Another is that the black matrix is reorganized and reconstructed in the LCD panel to obtain a high aperture ratio. The LCD panels each having a black matrix structure of a high aperture ratio are illustrated in FIGS. 2 and 3, as examples. Compared to that of FIG. 1, the LCD panels of FIGS. 2 and 3 have color filters and a black matrix particularly on the lower substrate.
In FIG. 2, an LCD panel having a high aperture ratio includes a lower substrate 10, an upper substrate 50, and a liquid crystal layer 30 interposed therebetween. On the lower substrate 10, a black matrix 52 that has an island shape is formed on a transparent substrate 5, and color filters 54 are also formed on the transparent substrate 5 and cover the black matrix 52. An overcoat layer 56 is disposed on the color filters 54 to protect and planarize the surface of the color filters 54. As contrary to the LCD panel of FIG. 1, the lower substrate 10 of FIG. 2 includes the black matrix 52 having an island shape, the color filters 54, and the overcoat layer 56. Further, a thin film transistor (TFT) “T” corresponding to the island shaped black matrix 52 is formed on the overcoat layer 56. Therefore, the black matrix and the color filters do not exist on the upper substrate 50. Also, a common electrode 58 is only disposed on a transparent substrate 5 of the upper substrate 50. However, an additional black matrix may be formed on the upper substrate 50 in this type.
In FIG. 3, a thin film transistor (TFT) “T” is formed on a transparent substrate 5 of the lower substrate 10. A passivation layer 22 covers the TFT “T” for protection. A black matrix 52 having an island shape is formed on the passivation layer 52 in the position corresponding to the TFT “T”. The color filters 54 are formed on the passivation layer 22 and cover the black matrix 52. An overcoat layer 56 is formed on the color filters 54 to protect the color filters 54 and planarize the surface of the lower substrate. A pixel electrode 26 is formed on the overcoat layer 56 and contacts a drain electrode 20b of the TFT “T” through a drain contact hole 24.
In the LCD panels shown in FIGS. 2 and 3, the black matrix 52 has an island shape and corresponds to the TFT “T”. Since both the black matrix 52 and the color filters 54 are formed on the lower substrate 10 and the black matrix 52 is very close to the TFT “T”, it is possible to make the black matrix 52 in a very small size. Namely, the black matrix 52 of FIGS. 2 and 3 does not have to cover the whole area of the TFT.
Actually, it is sufficient for the black matrix 52 to cover the active channel of the active layer 16 between the source 20a and drain 20b electrodes. However, as shown in FIG. 1, the black matrix formed in the upper substrate has a relatively larger size to cover the whole TFT, although the TFT does not need to be covered entirely. Therefore, the black matrix of FIG. 1 degrades an aperture ratio of the LCD panel.
The black matrix of the LCD panels illustrated in FIGS. 2 and 3 having a high aperture ratio is formed to be close to the TFT on the lower substrate. Therefore, the black matrix can precisely cover the TFT, especially the channel region of the active layer. As a result, the black matrix does not need to cover the whole area of the TFT, as shown in FIG. 1. The size of the black matrix shown in FIGS. 2 and 3 will be sufficient to cover the channel region of the active layer between the source and drain electrodes.
FIG. 4 is a partial cross-sectional view that illustrates a structure and a composition of the color filters and the black matrix.
In FIG. 4, the LCD panel having a high aperture ratio is simplified by illustrating only the transparent substrate 5, the black matrix 52, and the color filters 54. The black matrix 52 and the color filters 54 can be directly formed on the transparent substrate 5, as shown in FIG. 2. Alternatively, the color filters 54 and the black matrix 52 can be formed on the TFT that will be disposed at a region “K” between the transparent substrate 5 and the color filters 54, as shown in FIG. 3.
FIG. 5 is a plane view illustrating the color filters 54 and the black matrix 52 that are formed on a transparent substrate 5 to increase an aperture ratio. In FIG. 5, a rectangular black matrix 52a surrounds the color filters 54 having red (R) 54a, green (G) 54b, and blue (B) 54c colors. A plurality of island shaped black matrices 52b (shown in FIG. 6A) are formed within the stripe shaped color filters 54. An area where the rectangular black matrices 52a are disposed is a non-display area, such that it is finally covered by external cases when an LCD panel is installed to a laptop computer, for example. An area where the color filters are disposed is a display area. Therefore, the TFTs, the pixel electrodes, and the island shaped black matrices are disposed in the display area.
FIG. 6A is an enlarged plane view of a portion “A” of FIG. 5 and illustrates a plurality of black matrices. A plurality of island shaped black matrices 52b are formed on the lower substrate to increase an aperture ratio, as shown in FIGS. 2 and 3. Each island shaped black matrix 52b corresponds to each TFT to protect the active channel from the light. Alternatively, the island shaped black matrices 52b can be formed on the upper substrate of FIG. 1 as a modification of the stripe shaped black matrices to increase an aperture ratio.
FIG. 6B is also an enlarged plane view of the portion “A” of FIG. 5 when an arrangement of the black matrices is changed. Although the island shaped black matrices are not shown in FIG. 6B, they are disposed at the stripe shaped color filters 54 and each corresponds to the TFT. The rectangular black matrix 52a is referred to as an external black matrix, and the island shaped black matrix 52b is referred to as an internal black matrix.
In forming the color filters 54 and the black matrices 52, the black matrices 52 are formed first, and then, the color filters 54 representing red 54a, green 54b, and blue 54c colors are formed on the internal black matrices. Thus, the color filters 54 cover the internal black matrices, as shown in FIGS. 2 and 3. As a result, a light efficiency is due to the small size of the black matrix.
However, there are some problems in aligning color filters to the corresponding internal black matrices and pixel electrodes. As widely known, it is important to exactly align each R, G, and B color filter to each internal black matrix and pixel electrode. If a misalignment of the color filters occurs, the LCD device cannot display clear images. Especially, since the LCD device has both the rectangular external black matrix and the plurality of the island shaped internal black matrices, it is much more difficult to align the color filters into the prearranged positions.
Moreover, since each color filter 54a, 54b, and 54c should cover all the island shaped internal black matrices 52b when forming the color filters 54, the internal black matrices 52b cannot act as alignment marks. Therefore, alignment marks for aligning the color filters are required. Additionally, when the black matrix 52 occupies a small area of the transparent substrate, aligning the color filters 54 becomes extremely difficult. Thus, a misalignment of R, G, and B color filters 54a, 54b, and 54c occurs frequently.